The present invention relates to an overhead conveyor system comprising a rail, at least one carriage provided with wheels, which is adapted to run along the rail and support a load suspended from the carriage, a rotatable drive shaft extending along the rail and located perpendicularly above the rail, and a drive means which is arranged on each carriage and adapted to be brought into contact with the drive shaft and thus propel the carriage.
It is in many contexts an essential advantage to be able to convey goods above floor level, for example, in production or distribution plants. Overhead conveyor systems are already known for this purpose and are available in various designs.
It is particularly common to convey light goods, such as clothes on hangers, hanging down. Overhead conveyor systems intended for this type of goods, however, are not designed to carry heavy goods, typically weighing 100 kg.
WO95/25656 discloses an overhead conveyor which allows the carrying of heavy loads. The overhead conveyor comprises a track and carriages movable on the track and propelled by means of a threaded rotating spindle which is positioned above the track. The spindle is equipped with alternately threaded, driving portions and smooth, idle portions. The carriages are interconnected in pairs to form units, one of the two carriages of the units being in engagement with a driving portion of the spindle.
The driving performed in this manner implies a mechanically exactly defined connection between carriages and spindle, resulting in strict tolerance requirements.
Since the carriages are interconnected in pairs, the propulsion of these two carriages occurs uniformly. When a carriage enters a threaded portion, it is thus propelled by the other carriage which engages a threaded portion. Therefore the entering must take place without any displacement whatsoever so as to prevent tension and jamming between the engagement surfaces.
Besides, the mechanical connection means that if a carriage belonging to a unit leaves the track or gets stuck, the other carriage in the unit which engages the spindle makes the spindle stop.
It is already known to avoid a mechanical connection between a carriage and a drive shaft in an overhead conveyor, for example, from Applicant""s own publication WO97/45348. A cradle is pivotable on its point of suspension so that, when subjected on one side to a downward load, it is pressed upwards on its other side against a rotating drive shaft. On the other side, inclined wheels are arranged, which as the drive shaft rotates propel the cradle.
However, this construction is not intended for heavy loads. The moment which is generated when a load is applied to the cradle and which the device uses for its propulsion, causes a risk of damage to the equipment in case of excessive forces. Moreover, the compressive force acting on the drive shaft is directly proportional to the load, and in case of excessive load there is a risk that the inclined wheels are damaged.
An object of the present invention is to solve the above problems and provide an overhead conveyor which allows transport of very heavy loads and yet does not have the problematic mechanical connection as described above.
This object is achieved by an overhead conveyor system according to the preamble to claim 1, in which the drive means comprises at least one wheel inclined relative to the longitudinal direction of the drive shaft and having a contact surface which is spring-loaded against the drive shaft, the spring load of the drive means being essentially independent of the load suspended from the carriage.
During rotation of the drive shaft, the inclination of the drive wheels will cause the carriage to be propelled along the rail.
It constitutes an advantage that the major part of the forces acting on the carriage will be taken up by the rail. The compressive force acting on the drive shaft will be essentially independent of the load. The spring load acting on the drive wheels is preferably such that a certain downward flexing of the rail may be compensated for by the resilience. Therefore the propulsion is guaranteed independently of the load. According to an embodiment, a spring means is arranged to press the drive wheels against the drive shaft.
The rail preferably consists of two parallel rail elements, and the wheels of the carriage are preferably arranged in at least one pair, one wheel of the pair running on one rail element, and the other wheel of the pair running on the other rail element. In this way it is guaranteed that the carriage is not turned about its longitudinal axis. The load is suitably carried centrally in the carriage, all wheels being pressed with the same amount of force against the rail elements and stably keeping the carriage on the rail.
According to an embodiment of the invention, the drive shaft alternately has thick portions with a first, greater diameter, and narrow portions with a second, smaller diameter, which are arranged in series along the shaft. Under the narrow portions of the drive shaft, the contact between a carriage supported on the rail and the drive shaft is cancelled since the spring-loaded drive wheels do not reach up to the drive shaft. This means that under the narrow portions of the drive shaft the carriage is not driven, and that it is freely movable in the lateral direction without being obstructed by a contact with the drive shaft.
The transition between portions having different diameters can be continuous, preferably conical. This makes it easier for the drive shaft to begin the driving of a carriage as the carriage is brought into contact with a thick portion of the drive shaft.
Under a portion of the drive shaft with the second, smaller diameter, the rail may have a junction, in which at least one branch rail is connected to the rail. A second drive shaft with the first, greater diameter is suitably arranged above and along the branch rail, which drive shaft at its end nearest the junction tapers towards the end, preferably in a conical manner.
A carriage which is moved into the junction has no drive in this position but can, when moved out on the extension of the rail or on the branch rail, enter a thicker drive shaft portion, in which case the drive means of the carriage are brought into contact with the drive shaft and the carriage is propelled once more.
In the junction the rail can be provided with at least one switching element adapted to continuously join the rail elements of the rail with the rail elements of the branch rail. Like switch points, the junction can thus direct carriages in different directions.
According to the invention, the drive shaft which extends past a junction can thus be continuous, without interruptions, which restricts the number of necessary drive sources. This is a great advantage over overhead conveyor systems where the drive shaft is positioned under the supporting rail. In fact, in these systems the drive shaft must have a discontinuity in connection with each junction, thus requiring a plurality of drive sources.
According to a preferred embodiment, the drive means comprises at least two wheels which are inclined relative to the longitudinal direction of the drive shaft and adapted to be brought into contact with the drive shaft on each side of the centre axis of the drive shaft. Since there is one wheel on each side of the drive shaft, the contact between the drive shaft and the wheels will be more stable. Furthermore the carriage can be driven in the opposite direction with maintained performance.
Suitably the overhead conveyor system comprises at least two carriages, which act upon each other by means of forces directed along the drive shaft. This action can be accomplished, for example, by successive carriages being interconnected in pairs in conveying units. A carriage that has no contact with the drive shaft, for example when positioned in a junction, can thus be driven by being connected with a second carriage which is in contact with the drive shaft.